In the prior art, a focus method for a zoom lens is well known that moves the whole body of a first lens group having positive refractive power when there is a focal adjustment while moving another lens group. However, when the whole body of the first lens group is desired to be moved and focused on a very near object, the effective diameter of the first lens group invariably must be made larger in order to maintain a constant luminous flux at the perimeter of the image plane. This occurs because the first lens group is moved to the object side when focusing on a very near object. This becomes apparent in that the image becomes a wide angle image. Because the lens weight increases when the effective diameter of the first lens group becomes larger, this causes an inconvenience in the case of a zoom lens intended for use on a portable television camera.
As a technique for resolving this problem, the first lens group has been divided into three lens subgroups of negative, positive, and positive refractive power, respectively, in order from the object side. Methods that cause the intermediate subgroup of positive refractive power to move and focus are shown in, for example, Japanese Patent Publications 59-4686 and 6-242378.
Generally, a zoom lens of the inner focusing type moves the first lens group. As compared to a front focus zoom lens that performs the same focusing, the effective diameter of the first lens group becomes smaller than that of the corresponding front focus zoom lens, making it easy to miniaturize the entire body of the lens system when using a zoom lens of the inner focusing type.
On the other hand, a greater variety of photographing construction diagrams have recently been sought. Photographing of very near objects, which requires a wide angle change of the focal distance, has been desired. However, when a wide angle change is attempted, there is a tendency for the effective diameter of the first lens group to become larger.
Generally, when a zoom lens of the inner focusing type is employed, a miniaturization of the zoom lens becomes easy, as stated above. However, when a wide angle change is attempted, a problem develops in that it becomes difficult to obtain a high optical performance while attempting to miniaturize the zoom lens.